Method and apparatus for the electrolytic removal of metal



Nov. 23, 1965 B. H. WILKINSON METHOD AND APPARATUS FOR THE ELECTROLYTIC REMOVAL OF METAL Filed Feb. 5, 1962 3 Sheets-Sheet 2 lnvenlor M M Z/Mw 'Nov. 23, 1965 B. H. WILKINSON 3,219,564

METHOD AND APPARATUS FOR THE ELECTROLYTIC REMOVAL OF METAL Filed Feb. 5, 1962 3 Sheets-Sheet 5 W sma/a Q2 5ER3/4 J g L I44 U/l M3 2 705/05 f/ M5 IUUU /0/ q /04 m m $CR3/l M2 m m1 m0 1 \/Vo\/ SCRZ/l m C T sum/2 /50 Q5 Inventor WM};

B wwwyw A HorneyS I Nov. 23, 1965 B. H. WILKINSON 3,219,564

METHOD AND APPARATUS FOR THE ELECTROLYTIC REMOVAL OF METAL Filed Feb. 5, 1962 3 Sheets-Sheet l Inventor United States Patent 3,21%564 METHOD AND APPARATUS FOR THE ELEC- TROLYTHE REMOVAL OF METAL Bernard Hall Wilkinson, Glasgow, Scotland, assignor to Rolls-Royce Limited, Derby, England, a company of Great Britain Filed Feb. 5, 1962, Ser. No. 171,229 Claims priority, application Great Britain, Feb. 7, 1961, 4,615/61 14 Claims. (Cl. 204-143) This invention is concerned with a method and apparatus for the electrolytic removal of metal.

Apparatus for the electrolytic removal of metal, such as described in application No. 114,357 filed 2nd June 1961, may utilise a workpiece (the anode) and a cathode which are separated by a small gap, electrolyte being forced in operation through the gap under pressure. By using a suitable electrolyte, the metal from the anode does not form on the cathode, but is precipitated out from the electrolyte in the form of insoluble or very slightly soluble hydroxides after the electrolyte has passed through the gap. Consequently there is no change in the dimensions of the cathode and hence the spacing between the anode and cathode will tend to increase as the electrolysis proceeds.

The anode/cathode spacing may be maintained constant by varying for example the position of the cathode to maintain constant current flow between the anode and cathode. This procedure however assumes that the conductivity of the electrolyte remains constant. The conductivity of the electrolyte may change due for example to a change in its temperature or chemical constitution, and this may have a serious effect on the spacing between the workpiece and cathode if this spacing is controlled only on the criterion of the anode/ cathode current.

According to the present invention, there is provided a method for the electrolytic removal of metal from a workpiece, including directing a flow of electrolyte between the workpiece and a cathode, making the workpiece electrically positive relative to said cathode, deriving a first signal which varies with direct current flow between the workpiece and cathode, deriving a second signal which varies with the conductivity of the electrolyte, modifying said first signal in accordance with said second signal to compensate for changes in said first signal due to variations in the conductivity of the electrolyte, and maintaining the spacing between the workpiece and cathode constant in accordance with said modified first signal.

The term signal is used in this specification in its general sense, and denotes a variable which can take any suitable form, e.g. the position of a movable member, or an electrical voltage.

There are many diflerent ways of carrying out the method according to the invention. One simple way which does not involve the use of a servo system is to represent each signal by the position of a pointer on a dial, an operator then modifying said first signal in accordance with said second signal and effecting such adjustment to said spacing as is necessary to maintain the spacing constant.

Preferably however the method is carried out automatically using a servo system, said modified first signal serving as the input or control signal of the servo system. In this case said modified first signal represents deviations from a reference value which is the value of said modified first signal when the spacing between the workpiece and cathode is at a desired value, and the conductivity of the electrolyte is at a standard value. The deviations of the modified first signal from the refer- 3,219,564 Patented Nov. 23, 1965 ence value can be derived in any suitable way as will be apparent to those skilled in the art.

A convenient arrangement is to use an electro-mechanical servo system, the method including deriving said first and second signals in the form of DC. electrical voltages.

Preferably said second signal is derived by obtaining a direct measurement of the conductivity of the electrolyte, rather than by making an indirect measurement such as the temperature of the electrolyte which varies with the conductivity of the electrolyte. Thus the method may include deriving said second signal by connecting two electrodes in one arm of a Wheatstone bridge, the electrodes being disposed in the flowing electrolyte, applying an oscillating signal across the Wheatstone bridge, and rectifying the output from the Wheatstone bridge.

Also according to the present invention there is provided apparatus for the electrolytic removal of metal from a workpiece, the apparatus including means for directing a flow of electrolyte between the workpiece and a cathode, means for deriving a first signal which varies with direct current flow between the workpiece and cathode, and means for deriving a second signal which varies with the conductivity of the electrolyte, the spacing between the workpiece and cathode being adjustable so that said spacing can be maintained constant in accordance with said first signal as modified in accordance with said second signal to compensate for changes in said first signal due to variations in the conductivity of the electrolyte.

The apparatus may include apparatus for use in the electrolytic removal of metal, which is described in application No. 114,357 filed 2nd June 1961.

Preferably the apparatus includes means for automatically eifecting said modification of the first signal in accordance with said second signal, and a servo system responsive to the resulting modified first signal for automatically maintaining said spacing constant.

The means for deriving said second signal may comprise two electrodes connected in one arm of a Wheatstone bridge, the electrodes being disposed in operation in the flowing electrolyte, means for applying an oscillating signal across the Wheatstone bridge, and means for rectifying the output from the Wheatstone bridge.

There will now be described by way of example only three embodiments of the invention with reference to the diagrammatic drawings accompanying the specification in which:

FIGURE 1 shows an apparatus for the electrolytic removal of metal,

FIGURE 2 shows an enlarged perspective view of a workpiece (anode), and electrolyte supply chute, and a working electrode (cathode),

FIGURES 3 and 4 show two alternative forms of apparatus for electrolytic removal of metal.

The apparatus shown in FIGURE 1 comprises a tank 10 in which is supported a fixture 11 for carrying the workpiece 12 which is clamped in position by means of clamps 13 and 14. The workpiece, in this instance, is a rough forged turbine blade of nimonic alloy and it is desired to form an accurate aerofoil profile on the blade.

The Working electrode 15 is attached to an electrolyte chute 16 (see FIGURE 2) supported in a boxlike enclosure 17 carried on a rod 18 by which the electrode assembly can be moved vertically. The rod 18 slides in a guide 19 and is driven up and down by gearing 20, 21 in turn driven by an electric motor 22, the gear wheel 20 extending through a slot in the guide 19 and engaging rack teeth (not shown) formed along the rod 18.

The electrolyte is supplied to the electrolyte tank 10 by a flexible pipe 23 fed via pipe 27 by a high pressure pump 24. The chute is shaped so as to ensure that the electrolyte flows into the gap 32 between the electrode 15 and the workpiece 12.

The electrolyte flows back through the tank and drain 25 into a filter 26 from which it flows via pipe 27 and pump 24 to pipe 23. The electrolyte is preferably an aqueous solution of sodium chloride and is delivered from pump 24 at a pressure of the order of 100l50 lbs. per square inch.

A branch connection 28 including a chamber 29 is connected to the pipe 27 so that electrolyte flowing in the pipe 27 also flows through the branch connection 28. Electrodes 30, 31 are mounted in the chamber and are connected by leads 34, 35 in one arm of a Wheatstones bridge, the other arms of which are formed by resistors 36, 37 and 38. An oscillator 40 having a frequency of 1 k./s. is connected across two opposite corners of the bridge, and a resistor 41 is connected across the other two corners of the bridge by leads 42, 43 each of which includes a switch as indicated at 45, 46. The switches 45, 46 are mechanically interconnected as shown diagrammatically by the dash line 47.

A variable tap 50 on the resistor 41 is connected by way of a rectifier 51 and a resistor 52 to a lead 53, and one end of the resistor 41 is connected to a lead 54. A condenser 56 is connected between lead 54 and the junction between rectifier 51 and resistor 52 and a resistor 57 is connected between leads 53 and 54. A lead 60 which is connected to an adjustable voltage supply (not shown), is connected by way of a resistor 61 to the junction between resistors 52, 57. The value of resistor 57 is substantially greater than the values of resistors 52 and 61.

In operation, the pump 24 is started, and 12 volts D.C. is supplied via leads 70, 71 from a power supply unit 72, the workpiece 12 being the anode, and the electrode being the cathode. The electrode 15 is then lowered independently of the control system shown in FIGURE 1 until the current passing through leads 70, 71 is of the order of 50 amps, and is then very gradually lowered until the current increases to the order of 100-200 amps.

The power unit 72 includes a D.C. amplifier which after the initial positioning of electrode 15, feeds a first voltage signal representative of the current flow in leads 70, 71 to a servo control unit 73 via leads 75, 76.

The electrolyte flowing through pipe 27 also passes through the branch connection 28. When the switches 45, 46 are closed, the oscillator 40 produces an alternating voltage across resistor 41 whose magnitude is dependent on the conductivity of the electrolyte. A portion of this voltage is tapped off by the variable tap 50, rectified by the rectifier 51, and smoothed by the condenser 56 and resistor 52. The magnitude of the rectified smoothed voltage which appears across the resistor 57 will therefore be representative of the conductivity of the electrolyte. The sensitivity of the arrangement can be adjusted by varying the position of the variable tap 50, and a D.C. voltage is superimposed on the rectified voltage across the resistor 57 by way of lead 60 and resistor 61. The voltage across leads 53, 54, i.e. the second voltage signal, is combined in the servo control unit 73 with the voltage across leads 75, 76 to produce a modified first voltage signal. A servo amplifier may be used in the servo control unit 73 to amplify said first and/or said second voltage signal. The D.C. voltage applied to lead 60 is such that when the spacing between the cathode 15 and the workpiece 12 is at a desired value, and when the conductivity of the electrolyte is at a standard value, the voltage fed to the motor 22 from the servo control unit 73 is zero. The variable tap 50 is adjusted so that a change in the voltage between leads 75, 76 due to a change in the conductivity of the electrolyte is exactly compensated by the change in the voltage between leads 53, 54 which reflects said conductivity change.

The motor 22 will therefore automatically adjust the position of the cathode 15 to maintain the desired spacing between the cathode 15 and the workpiece 12, independently of any change in the conductivity of the electrolyte. It will be understood that the servo control unit 73 may be designed to have any suitable operating characteristic consistent with maintaining the stability of the system.

In the form of the invention shown in FIGURE 3 the working electrode is shown diagrammatically at 15a, and the workpiece is also shown diagrammatically at 12a in an electrolyte tank 1011. The workpiece 12a is connected to HT. positive and the working electrode 15a is connected through a resistor 77 to H.T. negative. Thus the voltage across resistor 77 will be representative of the current flowing between the electrode 15a and the workpiece 12a.

Opposite ends of the resistor 77 are connected to the control grid 78 of one of a pair of cathode-coupled triodes 79 and 80 which form a balanced servo amplifier. The anodes of triodes 79 and 80 are connected through respective resistors 81 and 82 to the split field windings 83 and 84 of a servo motor which controls the position of the working electrode 15a relative to the workpiece 12a. The junction of windings 83 and 84 is connected to I-I.T. positive.

The triode 80 has its control grid fed with a signal representative of electrolyte conductivity. This signal is fed from the variable tapping of a potentiometer 85. The signal is derived by use of the circuit shown on the left-hand side of FIGURE 3. A constant voltage transformer 86 has its input supplied with the mains voltage, which may be 250 volts A.C., and produces 12 volts across its secondary winding. One side of the secondary winding of the transformer 86 is connected via lead 87 to a ring electrode 88 in a conductivity cell or chamber 29a, through which the electrolyte flows as indicated by the arrows 95. At the other end of the conductivity cell is another ring electrode 89 which is connected via lead 90 to a rectifier 91 and also to a resistor 92. The resistor 92 has its other end joined to the secondary winding of the transformer 36. In parallel with the resistor 92 is a capacitor 93 and it is the voltage developed across this capacitor which is applied across the potentiometer 35 as a signal representing conductivity of the electrolyte. The resistance of the conductivity cell is made approximately equal to that of resistor 92.

The operation of this circuit is as follows:

If we assume that there is a state of equilibrium and there is 6 volts developed across resistor 77 (i.e. so that the grid voltage of valve 79 is 6 volts negative relative to the cathode voltage of valve 79) and we assume that there is then a change of temperature in the electrolyte which lowers the resistance of the electrolyte, the result would be that more current would flow through resistor 77 and the voltage on the control grid of valve 79 will decrease relative to the cathode voltage of valve 79.

At the same time as this change occurs the electrodes 88 and 39 will pass more current because of the temperature rise, and this will cause a higher voltage to be developed across the resistor 92. The circuit is designed so that the decrease in voltage thus arising on the control grid of valve 80 relative to its cathode voltage matches the decrease in voltage of the control grid 78 relative to the cathode voltage of valve 79, so that the valves 79, 80 continue to pass the same current and the voltages across the split field windings 83, 84 of the servo motor remain balanced, i.e. the servo motor remains balanced. It will be noted that the servo motor has been balanced by a change of voltage on the control grid of valve 80 and not by an actual movement of the servo motor, i.e. not by movement of the cathode 15a relatively to the workpiece 12a. Therefore, the gap between the cathode and the workpiece remains substantially constant. With this circuit constant current across the cathode/workpiece gap is not maintained but it is allowed to vary. However the gap size which controls the finished shape of the workpiece is controlled accurately so as to be maintained substantially constant.

Assuming that there is a change in the current flowing between the workpiece 12a and the cathode 15a which is not due to a change in the conductivity of the electrolyte, this will be reflected by a change in the voltage of grid 78 which will unbalance the servo amplifier. The servo motor will then move the cathode 15a in a direction to rebalance the servo-amplifier, when the gap between the work piece and cathode will again be at its desired value. It will be appreciated that the desired value of the gap can be adjusted by varying the position of the variable tapping of the potentiometer 85. In this embodiment the first signal which varies with direct current flow between the workpiece and cathode can be regarded as the magnetic field produced by the split field winding 83; the second signal which varies with the conductivity of the electrolyte can be regarded as the magnetic field produced by the split field winding 84; and the modified first signal can be regarded as the resultant magnetic field produced by both split field windings 83, 84. Alternatively said first signal, said second signal and said modified signal can be regarded respectively as the DC. voltage (vectorially) across the Winding 83, the DC voltage (vectorially) across the winding 84, and the sum (vectorially) of the voltages across the windings 83, 84.

In FIGURE 4, a workpiece and a working cathode electrode are shown respectively at 101) and 101, the latter being adjustable towards and away from the workpiece by a servo motor 103 which drives the cathode electrode through a variable speed gearbox 104.

An electrolyte circuit (not shown) for causing circulating electrolyte to flow between the workpiece and cathode electrode can be as shown in FIGURE 1. DC. power supply leads 106, 107 are connected across the workpiece and cathode electrode via a potentiometer 108, as shown, and an ammeter 110 and a voltmeter 111 are connected in the circuit to indicate the current flowing between, and the voltage across, the workpiece and cathode electrode.

A first DC. voltage signal which varies with direct current flow between the workpiece and cathode, and whose magnitude depends on the setting of the adjustable arm of the potentiometer 108, is applied to a chopper unit 114 via leads 115, 116. The chopper unit 114 breaks up this first signal into a pulsating signal which is fed to a modifying unit 118, which receives a second pulsating signal representative of the conductivity of the electrolyte passing between the workpiece and the cathode electrode, from a conductivity measuring unit denoted 121).

The modifying unit 118 produces an output signal which represents said first signal modified in accordance with said second signal to compensate for changes in the first signal due to variations in the conductivity of the electrolyte from a predetermined reference value, and the output signal is fed to a further modifying unit 122. In the latter, the output signal is modified in accordance with a pulsating gap reference signal fed to the modifying unit 122 from a potentiometer 123, whose adjustable arm is set in accordance with the constant gap it is desired to maintain in operation between the workpiece and cathode electrode. The modifying unit 122 then produces a control signal in lead 126 which represents deviations of said output signal (i.e. the modified first signal) from said gap reference signal. Sources of pulsating signals for the units 114 and 121), and for the potentiometer 123 are denoted by the references 4 which are individually surrounded by a ring, and can have a frequency of, for example, 400 c./ s. The outer ringed reference numerals 1 and 2 denote power supplies, the ringed references 1 being maintained at for example 24 volts D.C. positive with respect to the ringed references 2.

FIGURE 4 also shows circuitry in generalised diagrammatic form for effecting at the start of an electrolytic metal removing process, a rapid approach movement of the cathode electrode from a rest position towards the workpiece, followed by a slow approach movement until the workpiece/electrode, gap is at or near the desired constant value, and for effecting at the end of the process a rapid retract movement of the cathode electrode to the rest position.

This circuitry includes three relays, none of whose relay coils are shown, which relays are as follows:

(1) a rapid advance and retract relay controlling contacts SCR1/1 and SCRl/Z,

(2) a slow approach relay controlling contact SCR2/ 1, and

(3) a servo selector relay controlling contacts SCR3/ 1, SCR3/ 2, SCR3/ 3 and SCR3/ 4. The normal positions of the SCR contacts i.e. when their respective relay coils are deenergised, are shown in FIGURE 4. The means for controlling the relay coils will be discussed hereinafter.

When the circuit is in the state shown in FIGURE 4, initially the voltage at point 130 is determined by the position of the adjustable arm of a potentiometer 131, current flowing from lead 132 via the potentiometer 131, a rectifier 133, resistor 134, contact SCRZ/ 1 and contact SCRl/Z to lead 136. When it is desired to commence an electrolytic metal removing process, a push button switch (not shown) is operated which causes contact SCRl/ 1 to close and contact SCRl/Z to open. Current now flows from lead 132 via contacts SCR1/1 and SCRZ/ 1, resistor 134, condenser C and resistor to lead 136, so that the condenser commences to charge and the voltage of point 130 rises. The latter voltage is fed to a unit 142, which in effect compares it with a DC. voltage signal received from the arm 143 of a potentiometer 144, the arm 143 being mechanically connected as indicated by 145 to a cathode electrode 101 so that its position varies with the position of the cathode electrode. The arm of potentiometer 131 is set so that initially before the condenser C begins to charge, no output signal is produced by the unit 142 when the cathode electrode is in its desired rest position. When, however, the condenser C begins to charge, the voltage at point 130 rises above that of the arm 143, and the unit 142, produces an output representing this which is fed to a chopper unit 150. The latter receives a pulsating signal from a source as indicated by the ringed reference 4 connected thereto and converts its input signal from unit 142 into a pulsating output signal which passes via contact SCR3/ 2 and an amplifier 151 to the servo motor 103, causing the latter to rotate. Power from the source indicated by the ringed reference 1 which is connected to the potentiometer 144, also passes via contact SCR3/4 to a solenoid (not shown) in the gear box 104, which energises the solenoid, and causes a gear ratio to be selected so that the cathode electrode is driven at relatively high speed towards the workpiece. The speed of movement of the cathode electrode depends on the magnitude of the difference between the voltage at point 130 and the voltage of arm 143, and the cathode electrode is driven towards the workpiece since the voltage of point 130 is increasing and is normally at any given moment during this initial approach movement, greater than the voltage of arm 143. As the cathode electrode moves forwardly, the voltage of arm 143 rises. Should the voltage of arm 143 rise above that of point 130 for any reason, the servo motor 103 will reverse temporarily causing the cathode electrode to retract until the voltage of point 130 is again greater than that of the arm 143, when the cathode electrode will again resume its forward movement. A tachometer signal derived from a tachometer (not shown) associated with the servo motor 103 is fed to the amplifier 151 for modifying the operating characteristic of the servo system in known manner.

When the cathode electrode has completed its rapid approach movement, e.g. when the gap between the workpiece and cathode electrode has fallen to 0.030 in., contact SCRZ/ 1 opens so that the condenser C now charges at a slower rate via resistors 137 and 134. As a result,

the cathode electrode now moves more slowly towards the workpiece.

When the gap between the workpiece and the cathode electrode is at or near the desired constant value which it is desired to maintain, contacts SCR3/lt and SCR3/3 close, and contacts SCR3/2 and SCR3/4 open. Thus the control signal from unit 122 is now fed via lead 126, contact SCR3/1, and amplifier 151 to the servo motor 103, so as to control the latter, and the closing of contact SCR3/3 and the opening of contact SCR3/4 causes a lower gear ratio to be selected in gearbox 104, so that the servo motor 103 must rotate a greater number of times than previously to move the cathode electrode through a given distance.

The electrolytic metal removing process is then continued whilst the gap between the workpiece and cathode electrode is maintained constant at the desired value by the servo system.

When the workpiece achieves the desired form all the SCR contacts are caused to revert to their settings shown in FIGURE 4. The voltage of point 130 then begins to fall because the condenser C discharges via resistor 134 and contacts SCRZ/l and SCRl/Z, and the voltage of point 136 is lower than the voltage of arm 143. Thus the servo motor is driven in the reverse direction and retracts the cathode electrode 101 rapidly, since the higher gear ratio is selected in the gear box 104 by the closing of contact SCR3/4. The cathode electrode therefore returns to its rest position when the condenser C has discharged, and the voltage of point 130 becomes steady and is equalled by the voltage of arm 143.

It will be appreciated that there are many different ways of controlling the SCR contacts so that the system operates as described above. Thus apart from controlling these contacts by a timing mechanism, which would not be very satisfactory, the contacts could be controlled by micro switches, two of which are diagrammatically indicated at 160, which are arranged to be operated sequentially by the cathode electrode as the latter moves forwardly.

If desired, the opening of contact SCRZ/l at the commencement of the slow approach movement of the cathode electrode, and the operation of the various SCR3 contacts at the commencement of an electrolyte metal removing process, could be controlled by relays responsive to the current flowing between the workpiece and cathode electrode.

Desirably where there is a common power supply feeding the sources denoted by ringed references 4, an under volt relay U/V(S) (not shown) is associated with this common power supply so as to be responsive to the voltage thereof. Furthermore an under volt relay U/ V (H) is desirably connected between the workpiece and cathode electrode. The contacts of such relays are then connected in the circuitry so that during an electrolytic metal re moving process, should the relay U/V (H) sense an undesired drop in voltage, the cathode electrode will be retracted to its rest position, and will be automatically moved forward again when the full voltage is restored, and should the relay U/V(S) sense an undesired drop in voltage the cathode electrode will retract to its rest position and remain there, even though the voltage to which it is responsive rises again. In the latter case, the previously mentioned push button (not shown) must be operated to cause the cathode electrode to move forward again.

If desired each chopper unit 114 and 150 can incorporate a succeeding amplifier stage. An alternative arrangement in which only one chopper unit is necessary would be to derive the first signal (which varies with current flow between the workpiece and cathode electrode), the second signal (which varies with conductivity of the electrolyte), and the gap reference signal in the form of DC. voltages, and to combine these signals into a DC. voltage control signal in lead 126. The one chopper unit would then be arranged preceding the amplifier 151 and would receive a DC. signal in operation either via contact SCR3/ 2 or contact SCR3/ 1. With this arrangement, if the second signal was initially derived as an alternating signal, some suitable arrangement such as a rectifier would be necessary to convert it into a DC signal.

The second signal can be obtained initially in alternating form (to suit the circuit shown in FIGURE 4), by means of two axially spaced electromagnetic coils disposed in the electrolyte, eg in the conduit 28 of FIGURE 1. One of these coils serves as a primary coil and is energised With 9 volts A.C. at 400 c./s. The second signal is taken as the resulting voltage induced in the second coil, the magnetic coupling between the coils increasing as the conductivity of the electrolyte increases.

It will be appreciated that many modifications and variations may be made to the embodiments that have been described without departing from the scope of the invention. Thus, for example, although the apparatus shown in FIGURES 1 and 2 is concerned with the production of a concave surface on the workpiece 12, the apparatus can be adapted to perform many other operations on various types of workpiece e.g. forming a hole or slot in a workpiece.

I claim:

1. A method for the electrolytic removal of metal from a workpiece, including directing a flow of electrolyte between the workpiece and a cathode, making the workpiece electric-ally positive relative to said cathode, deriving a first electrical signal the value of which varies with direct current flow between the workpiece and cathode, deriving a second electrical signal the value of which varies with the conductivity of the electrolyte, modifying said first signal in accordance with said second signal to compensate for changes in the value of said first signal due to variations in the conductivity of the electrolyte, and adjusting the spacing between the workpiece and cathode in response to the value of said modified first signal so that the spacing is maintained substantially constant.

2. A method for the electrolytic removal of metal from a workpiece, including directing a flow of electrolyte between the workpiece and a cathode, making the workpiece electrically positive relative to said cathode, deriving a firs-t electrical signal the value of which varies with direct current flow between the workpiece and cathode, deriving a second electrical signal the value of which varies with the conductivity of the electrolyte, modifying said first signal in accordance with said second signal to compensate for changes in the value of said first signal due to variations in the conductivity of the electrolyte, and automatically maintaining the spacing between the workpiece and cathode constant in accordance with said modified first signal using a servo system, said modified first signal serving as the control input signal of the servo system.

3. A method for the electrolytic removal of metal from a workpiece, including directing a flow of electrolyte between the workpiece and a cathode, making the workpiece electrically positive relative to said cathode, driving a first signal in the form of a DC. electrical voltage which varies with direct current fiow between the workpiece and cathode, deriving a second signal in the form of a DC. electrical voltage which varies with the conductivity of the electrolyte, modifying said first signal in accordance with said second signal to compensate for changes in said first signal due to variations in the conductivity of the electrolyte, and automatically maintaining the spacing between the workpiece and cathode constant in accordance with said modified first signal using an electro-mechanical servo system, said modified first signal serving as the control input signal of the servo system.

4. A method for the electrolytic removal of metal from a workpiece, including directing a flow of electrolyte between the workpiece and a cathode, making the workpiece electrically positive relative to said cathode, deriving a first signal in the form of a DC. electrical voltage which varies with direct current flow between the workpiece and cathode, obtaining a direct measurement of the conductivity of the electrolyte in the form of a second D.C. electrical voltage signal, modifying said first signal in accordance with said second signal to compensate for changes in said first signal due to variations in the conductivity of the electrolyte, and automatically maintaining the spacing between the workpiece and cathode constant in accordance with said modified first signal using an electromechanical servo system, said modified first signal serving as the control input signal of the servo system.

5. A method for the electrolytic removal of metal from a workpiece, including directing a flow of electrolyte between the workpiece and a cathode, making the work piece electrically positive relative to said cathode, deriving a first signal in the form of a DC. electrical voltage which varies with direct current flow between the workpiece and cathode, obtaining a direct measurement of the conductivity of the electrolyte in the form of a second D.C. electrical voltage signal by connecting two electrodes in one arm of a Wheatstone bridge, disposing the electrodes in the flowing electrolyte, applying an oscillating signal across the Wheatstone bridge, and rectifying the output from the Wheatstone bridge, modifying said first signal in accordance with said second signal to compensate for changes in said first signal due to variations in the conductivity of the electrolyte, and automatically maintain-- ing the spacing between the workpiece and cathode constant in accordance with said modified first signal using an electro-rnechanical servo system, said modified first signal serving as the control input signal of the servo system.

6. A method for the electrolytic removal of metal from a workpiece, including directing a flow of electrolyte between the workpiece and a cathode, making the workpiece electrically positive relative to said cathode, deriving a first signal which varies with direct current flow between the workpiece and cathode, deriving a second signal in the form of an A.C. electrical voltage which varies with the conductivity of the electrolyte, modifying said first signal in accordance with said second signal to compensate for changes in said first signal due to varitions in the conductivity of the electrolyte, and automatically maintaining the spacing between the workpiece and cathode constant in accordance with said modified first signal using a servo system, said modified first signal serving as the control input signal of the servo system.

7. A method for the electrolytic removal of metal from a workpiece, including directing a flow of electrolyte between the workpiece and a cathode, making the workpiece electrically positive relative to said cathode, deriving a first signal which varies with direct current flow between the workpiece and cathode, deriving a second signal in the form of an A.C. electrical voltage which varies with the conductivity of the electrolyte, by electromagnetically inducing an AC. voltage in a coil, the electromagnetic coupling with which depends on the conductivity of the electrolyte, modifying said first signal in accordance with said second signal to compensate for changes in said first signal due to variations in the conductivity of the electrolyte, and automatically maintaining the spacing between the workpiece and cathode constant in accordance with said modified first signal using a servo system, said modified first signal serving as the control input signal of the servo system.

8. Apparatus for the elecrolytic removal of metal from a workpiece, including means for mounting an anode workpiece and a cathode so that they are relatively movable towards and away from each other, means for directing a flow of electrolyte between the workpiece and cathode, means deriving a first signal which varies with direct current flow between the workpiece and cathode, means deriving a second signal which varies with the conductivity of the electrolyte, means automatically modifying said first signal in accordance with said second signal to compensate for changes in said first signal due to variations in the conductivity of the electrolyte, and means responsive to the modified first signal which automatically maintains the spacing between the workpiece and cathode constant. 7

9. Apparatus for the electrolytic removal of metal from a workpiece, including means for mounting and anode workpiece and a cathode so that they are relatively movable towards and away from each other, means for directing a flow of electrolyte between the workpiece and cathode, means deriving a first D.C. electrical voltage signal which varies with direct current flow between the workpiece and cathode, means providing a direct measurement of the conductivity of the electrolyte in the form of a second D.C. electrical voltage signal, means automatically modifying said first signal in accordance with said second signal to compensate for changes in said first signal due to variations in the conductivity of the electrolyte, and an electromechanical servo system responsive to the modified first signal which automatically maintains the spacing between the workpiece and cathode constant.

10. Apparatus for the electrolytic removal of metal from a workpiece, including means for mounting an anode workpiece and a cathode so that they are relatively movable towards and away from each other, means for directing a flow of electrolyte between the workpiece and cathode, means deriving a first D.C. electrical voltage signal which varies with direct current flow between the workpiece and cathode, a Wheatstones bridge, two electrodes connected in one arm of the Wheatstone bridge and disposed in the flowing electrolyte, means applying an oscillating signal across the Wheatstones bridge, and means rectifying the output from the Wheatstone bridge to provide a second D.C. electrical voltage signal which is a direct measurement of the conductivity of the electrolyte, means automatically modifying said first signal in accordance with said second signal to compensate for changes in said first signal due to variations in the conductivity of the electrolyte, and an electro-mechanical servo system responsive to the modified first signal which automatically maintains the spacing between the workpiece and cathode constant.

11. Apparatus for the electrolytic removal of metal from a workpiece, including means for mounting and anode workpiece and a cathode so that they are relatively movable towards and away from each other, means for directing a flow of electrolyte between the workpiece and cathode, means deriving a first electrical signal which varies with direct current flow between the workpiece and cathode, two coils so disposed in the flowing electrolyte as to have a mutual inductance which depends on the conductivity of the electrolyte, means energising one of said coils with an alternating voltage, the resulting voltage induced in the other coil being taken as a second signal which varies with the conductivity of the electrolyte, means automatically modifying said first signal in accordance with said second signal to compensate for changes in the first signal due to variations in the conductivity of the electrolyte, and means responsive to the modified first signal which automatically maintains the spacing between the workpiece and cathode constant.

12. Apparatus for the electrolytic removal of metal from a workpiece, including means for mounting an anode workpiece and a cathode so that they are relatively movable towards and away from each other, means for directing a flow of electrolyte between the workpiece and cathode, means deriving a first signal which varies with direct current flow between the workpiece and cathode, means deriving a second signal which varies with the conductivity of the electrolyte, means automatically modifying said first signal in accordance with the said second signal to compensate for changes in said first 1.1 signal due to variations in the conductivity of the electrolyte, means responsive to the modified first signal which automatically maintains the spacing between the workpiece and cathode constant, and automatic control means for controlling relative movement of the workpiece and cathode towards one another from a position in which they are relatively widely spaced to a position in which the gap therebctween is at the desired constant gap width.

13. Apparatus as claimed in claim 12 in which said automatic control means is adapted to control relative movement of the workpiece and cathode towards one another at a slower rate, when the gap width is relatively small, than when it is relatively large.

14. Apparatus as claimed in claim 12 in which said automatic control means is also adapted to control relative movement of the workpiece and cathode away from one another from a position in which the gap therebetween is at the desired constant gap width to a position in which the gap is relatively large.

References Cited by the Examiner UNITED STATES PATENTS JOHN H. MACK, Primary Examiner.

JOHN R. SPECK, Examiner. 

1. A METHOD FOR THE ELECTROLYTIC REMOVAL OF METAL FROM A WORKPIECE, INCLUDING DIRECTING A FLOW OF ELECTROLYTE BETWEEN THE WORKPIECE AND A CATHODE, MAKING THE WORKPIECE ELECTRICALLY POSITIVE RELATIVE TO SAID CATHODE, DERIVING A FIRST ELECTRICAL SIGNAL THE VALUE OF WHICH VARIES WITH DIRECT CURRENT FLOW BETWEEN THE WORKPIECE AND CATHODE, DERIVING A SECOND ELECTRICAL SIGNAL THE VALUE OF WHICH VARIES WITH THE CONDUCTIVITY OF THE ELECTROLYTE, MODIFYING SAID FIRST SIGNAL IN ACCORDANCE WITH SAID SECOND SIGNAL TO COMPENSATE FOR CHANGES IN THE VALUE OF SAID FIRST SIGNAL DUE TO VARIATIONS IN THE CONDUCTIVITY OF THE ELECTROLYTE, AND ADJUSTING THE SPACING BETWEEN THE WORKPIECE AND CATHODE IN RESPONSE TO THE VALUE OF SID MODIFIED FIRST SIGNAL SO THAT THE SPACING IS MAINTAINING SUBSTANTIALLY CONSTANT. 